A radio signal transmitted from or to a mobile or portable telephone is affected by a number of factors. For instance, the power of a radio signal transmitted from an antenna decays as a function of the distance from the transmitter. In addition, there are usually obstacles In the radio signal transmission path such that radio waves may partly be obstructed or absorbed by the physical environment around the propagation path. A radio wave may also be reflected from the terrain, fixed or mobile objects in the propagation path, such as vehicles, or from a discontinuity in the atmosphere. In certain instances, a reflected signal is significantly attenuated whereas in some other instances most of the radio energy is reflected, and only part is absorbed. Such reflections generates a plurality of different propagation paths for the radio signal between a transmitter and a receiver, and this phenomenon is called multiple road propagation (or multipath propagation). The reflection and the multipath propagation give rise to the "bending" of radio waves around corners and the propagation beyond hills and buildings becomes possible, as well as into multilevel car parks and tunnels.
Multipath propagation causes very difficult problems in the vicinity of a mobile phone. Three main problems being the delay spread of the received signal, the Rayleigh fading caused in the strength of a received signal by varying phase shift between different paths and the varying frequency modulation due to the Doppler shift between various propagation paths. The first mentioned phenomenon is due to the fact that since the propagation path of the reflected signals is longer than the direct path (from the transmitter to the receiver, e.g. from the base station to the mobile station) which gives rise to signal delays. Since various paths lead to slightly different arrival times, the signal spreads. Rayleigh fading is caused by the phase and amplitude of the reflected radio waves relative to the phase of a directly advancing wave being different, thus attenuating the radio message at the receiving end. If the receiver receives e.g. two signals propagated along two different paths, having phase difference of 180.degree., they cancel each other in the receiver, so that the signal weakens or disappears entirely. The last mentioned phenomenon, i.e. Doppler shift, is caused by the movement of a telephone, a vehicle or a reflecting object in relation to the transmitter and/or the receiver (e.g. base station), whereby both of the mean frequencies of the received reflected signal and of the directly propagated signal deviate from the mean frequency of the transmitted signal by a different amount and from a different direction. The accidental modulation caused by such changes results in a transmitted frequency being audible as unpleasant crackling or whistling by the user.
Owing to the great number of different factors affecting the propagation of a RF channel signal, particularly of a multipath channel, the RF channel has already to date been studied and simulated thoroughly. J. D. Parsons examines in his book The Mobile Radio Propagation Channel, Pentech Press Limited, London, (ISBN 0-7273-1316-9) the properties and simulation methods of the RF channel of a mobile station system. FIG. 6.9 on page 182 of the book demonstrates the scattering function of a radio signal within a range in which powerful multipath propagation occurs. The figure shows a vivid example of the relations between the received power, time delay and the Doppler shift. The figure demonstrates how a different Doppler shift occurs in different routes in the multipath circumstances, said shift being both positive and negative. According to Parsons, the dominating factors causing scattering can be identified by interpreting the Doppler shift as a function of the space angle of the received signal. Similarly, a physical image can be created on the propagation mechanisms in said range.
It is necessary, for instance, to be able to simulate the real properties of the radio path described above when testing the apparatus and in prototype tests using a reliable and realistic method corresponding to real life. A good testing means can thereafter be used also as a tool in developing new systems, coding and modulation methods, and in estimating the correction and diversity methods. When selecting a test method, typically a decision is made whether to use a program-based and/or hardware-based simulation. Various procedures are available for simulation both for narrow-band and wide-band channels.
A known simulator for a RF channel based on the use of an attenuator is presented in FIG. 1. A radio signal is coupled from the input in via an attenuator 11 to an attenuator 13 controlled by a fading generator 12. The output of attenuator 13 is coupled via an amplifier or an attenuator 14 to the output Out. An advantage of the simulator is that it is simple and inexpensive to manufacture, and when implemented passively it also operates in two directions. The design is appropriate for the simulation of fading by changing the attenuation of the attenuator 13 on the basis of the control 12. A drawback is that while being analogue it includes severe inaccuracies, and is not appropriate for simulation of multipath propagation or Doppler shift effects.
FIG. 2 presents a Doppler simulator in which the basic coupling is provided for the simulation of one propagation path, the dotted portion showing how by parallel coupling a simulator can be provided to simulate two propagation paths. Equally, a simulator for several propagation paths can be obtained by coupling several branches in parallel, each branch having different coefficients A and .omega.. A radio signal is coupled from the input RFin into two branches, where the mixer 21 of branch 1 is controlled at frequency Fm developed by oscillator 24 and the mixer 22 of branch Q at the frequency Fm phase-shifted by the 90.degree. phase shifting means 23. The mixing results 210 and 220 obtained from the mixers 21, 22 are filtered in low-pass filters 211 and 221, as the outputs of which the base-frequency carrier-wave vectors I (reference 212) and Q (reference 222) are produced. One propagation path is illustrated with the multiplier pairs 25 and 26, in which a signal of the propagation path is multiplied with the attenuation A.sub.1 (t) of the propagation path and the Doppler shift of the frequency is illustrated by multiplying a signal of the I branch with cos(.omega..sub.1 t) and a signal of the Q branch with sin (.omega..sub.1 t). In other words, the combined effect of the attenuation A.sub.1 and the Doppler shift .omega..sub.1 is illustrated with coefficients A.sub.1 cos(.omega..sub.1 t) and A.sub.1 sin(.omega..sub.1 t). Respectively, the attenuation A.sub.2 and the Doppler shift .omega..sub.2 in the other branch are illustrated with coefficients A.sub.2 cos(.omega..sub.2 t) and A.sub.2 sin(.omega..sub.2 t). The signals entering along different branches are summed in adders 252 and 262 and multiplied in multipliers 27, 28 with mixing frequencies Fm and the 90.degree. phase-shifted component Fm 90 thereof, and the outputs obtained from the mixers 27, 28 are summed in the adder 29, in the output OUT whereof the original RF input signal is thus produced, being attenuated with different propagation constants A.sub.1 and A.sub.2 and Doppler shifted by angle frequencies .omega..sub.1 and .omega..sub.2.
This kind of simulation of an RF channel implements the attenuations of different propagation paths and the Doppler frequencies of the carrier wave frequency. However, the Doppler shift produced in the modulation cannot be presented as a template, neither can the actual delay of the channel nor the delay difference of various propagation paths that is, the multiple delay spread. For such simulations a tapped filter structure is used and, if needed, is added into the I and Q components of each branch. An FIR structure showing the template of the multipath propagation is presented in FIG. 3. Therein, the I or Q component of each branch is coupled as an input signal 31 into a delay chain composed of N unit delays 32, in each whereof the signal is delayed by one unit delay .tau.. The multipath propagation is illustrated by summing the delayed signals with different weight coefficients a.sub.0 -a.sub.N in the multipliers 33 and adder 34. The output of the tapped filter gives the input signal delayed by a plurality of different delays, each different delays weight by coefficients a.sub.0 -a.sub.N.
By combining the designs of FIGS. 2 and 3, a multiple path--Doppler simulator for the RF channel can be implemented. The designs of FIGS. 2 and 3 can be combined as such or so that the multiplication corresponding to the Doppler shift is performed separately for each coefficient a.sub.0 of the FIR branch shown in FIG. 3 and the separate multipliers 25 and 251, etc. of FIG. 2 are replaced with such FIR branches. Which of the combination designs is more advantageous to be implemented is dependent on how many multipath branches and how many FIR pins are provided in the simulator, and also how large are the Doppler shifts which it is desired to be simulated and at which precision the simulation is to take place. The method shown in FIGS. 2 and 3 is needed for simulating the effects of the multipath in currently used mobile phone systems, such as GSM (Global System for Mobile Communications), JDC (Japan Digital Cordless), ADC (American Digital Cellular), etc. Analogue inaccuracies are involved in the method, unless implemented prior to the DSP (Digital Signal Processing). The bandwidth is also confined to the range of 1 MHz.
An I/Q multipath simulator for an RF channel can be produced by coupling in parallel several of the apparatus shown in FIGS. 2 and 3. An advantage of said procedure is a good performance when the bandwidth or other factors are fixed and located in an appropriate range. The simulator can be implemented analogue or digital. Of the drawbacks, the great number of means and analogue inaccuracies may be mentioned, being caused by the conversion of the signals into I and Q components, equivalent to the simulator shown in FIG. 2. The implementation of such arrangement becomes difficult if the parameters are varied within a wide range.
With the FIR simulator as shown in FIG. 3 the radio channel can be simulated with the Doppler phenomenon also. This kind of simulator can be implemented either as a hardware design with a digital signal processor (DSP) or as a hard wired logic design. A drawback of the apparatus design lies in the great number of components required by several delay lines and multipliers, and moreover, this simulator must have a complicated algorithm for simulating Doppler shifts. With fixed delays the delay resolution accuracy is poor, and the unit delay .tau. corresponds to a Doppler shift corresponding to movement speeds over 100 m/s. This can be reduced to some extent so that instead of whole delay steps, the signal is delayed by changing the weight coefficients ai in the FIR block so that instead of the signal being delayed by one delay unit, it is slightly deformed and delayed by only a fraction of the tap delay. If, e.g. the weight factors are expressed with 10 bits, a delay resolution of T/10 to T/50 is achievable. If a tapping delay is e.g. 50 ns, and the signal is expressed to an accuracy of 10 bits or 1000 levels and the output is allowed to deform so that it is accurate to within 20 signal levels or 1/50th of the original, the smallest shift of the apparent time axis of the output signal is approximately 1 ns, which is too long, considering a Doppler simulation in practice. This is achievable only if the tapping factors are changed at the sampling frequency. This results in a complex structure with fast multipliers and a high power consumption. Therefore, each branch would require a separate Doppler shift unit, as in FIG. 2, and correspondingly, more computation. In the preceding example, the carrier wave f.sub.c and the modulation are dealt with as differing from one another.
It is frequently necessary to examine also the impact of the propagation delay of a radio wave on the functioning of the system. For instance, the control of the radio volume between a fixed base station and a mobile subscriber apparatus needed in a Code Division Multiple Access (CDMA) communication depends both on the features of the RF channel described above (attenuation, multipath propagation) and on the actual propagation delay of the RF channel, which the simulators described above were not able to produce.